Wankel engine

This article is about a particular pistonless rotary engine. For other pistonless rotary engines, see pistonless rotary engine. For piston designs arranged in a rotary configuration, see rotary engine.

The engine was invented by German engineer Felix Wankel. Wankel received his first patent for the engine in 1929, began development in the early 1950s at NSU, and completed a working prototype in 1957.[1] NSU subsequently licensed the design to companies around the world, which have continually added improvements.

The first DKM Wankel engine designed by Felix Wankel, the DKM 54 (Drehkolbenmotor), at the Deutsches Museum in Bonn, Germany: the rotor and its housing spin.

The first KKM Wankel Engine designed by Hanns Dieter Paschke, the NSU KKM 57P (Kreiskolbenmotor), at Autovision und Forum, Germany: the rotor housing is static.

In 1951, the German engineer Felix Wankel began development of the engine at NSU Motorenwerke AG. Two engines were developed. The first was by Felix Wankel, the DKM motor. The second, the KKM motor, was developed by Hanns Dieter Paschke, which was adopted forming the modern Wankel engine.[2] The Wankel engine design used today was not designed by Felix Wankel. Titling the engine 'the Paschke engine' would be more apt.

The basis of the DKM type of motor is that both the rotor and the housing spin around on separate axes. The DKM motor reached higher revolutions per minute and was more naturally balanced. However, the engine needed to be stripped to change the spark plugs and contained more parts. The KKM engine is simpler, having a fixed housing.

The first working prototype, DKM 54, produced 21 horsepower and ran on February 1, 1957, at the NSU research and development department Versuchsabteilung TX.[1][3] The KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) was constructed by NSU engineer Hanns Dieter Paschke in 1957 without the knowledge of Felix Wankel, who remarked "you have turned my race horse into a plow mare".[4]

In 1960, NSU, the firm that employed the two inventors, and the US firm Curtiss-Wright, signed an agreement, with NSU concentrating on low- and medium-powered Wankel engine development and Curtiss-Wright developing high-powered Wankel engines, including aircraft engines of which Curtiss-Wright had decades of experience designing and producing.[5] Curtiss-Wright recruited Max Bentele to head their design team.

Citroën did much research, producing the M35 and GS Birotor, using engines produced by Comotor, a joint venture of Citroën and NSU. General Motors seemed to have concluded the Wankel engine was slightly more expensive to build than an equivalent reciprocating engine. General Motors claimed to have solved the fuel economy issue, but failed in obtaining acceptable exhaust emissions. Mercedes-Benz fitted a Wankel engine in their C111 concept car.

In 1961, the Soviet research organization of NATI, NAMI, and VNIImotoprom started experimental development, and created experimental engines with different technologies.[8] Soviet automobile manufacturer AvtoVAZ also experimented in Wankel engine design without a license, introducing a limited number of engines in some cars.[9]

Despite much research and development throughout the world, only Mazda has produced Wankel engines in large numbers.

In Britain, Norton Motorcycles developed a Wankel rotary engine for motorcycles, based on the Sachs air-cooled rotor Wankel that powered the DKW/Hercules W-2000 motorcycle, which was included in their Commander and F1. Norton improved on the Sachs's air cooling, introducing a plenum chamber. Suzuki also made a production motorcycle powered by a Wankel engine, the RE-5, using ferroTiC alloy apex seals and an NSU rotor in a successful attempt to prolong the engine's life.

Mazda and NSU signed a study contract to develop the Wankel engine in 1961 and competed to bring the first Wankel-powered automobile to market. Although Mazda produced an experimental Wankel that year, NSU was first with a Wankel automobile for sale, the sporty NSU Spider in 1964; Mazda countered with a display of two- and four-rotor Wankel engines at that year's Tokyo Motor Show.[1] In 1967, NSU began production of a Wankel-engined luxury car, the Ro 80.[10] However, NSU had not produced reliable apex seals on the rotor, unlike Mazda and Curtiss-Wright. NSU had problems with apex seals' wear, poor shaft lubrication, and poor fuel economy, leading to frequent engine failures, which led to large warranty costs curtailing further NSU Wankel engine development. This premature release of the new Wankel engine gave a poor reputation for all makes and even when these issues were solved in the last engines produced by NSU in the second half of the '70s, sales did not recover.[1]Audi, after the takeover of NSU, built in 1979 a new KKM 871 engine with side intake ports and 750 cc per chamber, 170 HP at 6,500 rpm, and 220 Nm at 3,500 rpm. The engine was installed in an Audi 100 hull they named "Audi 200", but the engine was not mass-produced.

Mazda, however, claimed to have solved the apex seal problem, and operated test engines at high speed for 300 hours without failure.[1] After years of development, Mazda's first Wankel engine car was the 1967 Cosmo 110S. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation. However, Mazda chose a method to comply with hydrocarbonemission standards that, while less expensive to produce, increased fuel consumption. Unfortunately for Mazda, this was introduced immediately prior to a sharp rise in fuel prices. Curtiss-Wright produced the RC2-60 engine which was comparable to a V8 engine in performance and fuel consumption. Unlike NSU, by 1966 Curtiss-Wright had solved the rotor sealing issue with seals lasting 100,000 miles.[11]

Mazda later abandoned the Wankel in most of their automotive designs, continuing to use the engine in their sports car range only, producing the RX-7 until August 2002. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1991 Eunos Cosmo sports car. In 2003, Mazda introduced the Renesis engine fitted in the RX-8. The Renesis engine relocated the ports for exhaust from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. Some early Wankel engines had also side exhaust ports, the concept being abandoned because of carbon buildup in ports and the sides of the rotor. The Renesis engine solved the problem by using a keystone scraper side seal, and approached the thermal distortion difficulties by adding some parts made of ceramics.[12] The Renesis is capable of 238 hp (177 kW) with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines,[13] all from a nominal 1.3 L displacement. However, this was not enough to meet more stringent emissions standards. Mazda ended production of their Wankel engine in 2012 after the engine failed to meet the improved Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle.[14] The company is continuing development of the next generation of Wankel engines, the 16X. Mazda announced the introduction of the series-hybrid Mazda2 EV car using a Wankel engine as a range extender.[15]

American Motors (AMC) was so convinced "... that the rotary engine will play an important role as a powerplant for cars and trucks of the future....", that the chairman, Roy D. Chapin Jr., of the smallest U.S. automaker signed an agreement in February 1973, after a year's negotiations, to build Wankels for both passenger cars and Jeeps, as well as the right to sell any rotary engines it produced to other companies.[16][17] American Motors' president, William Luneburg, did not expect dramatic development through to 1980, however Gerald C. Meyers, AMC's vice president of the product (engineering) group, suggested that AMC should buy the engines from Curtiss-Wright before developing its own Wankel engines and predicted a total transition to rotary power by 1984.[18] Plans called for the engine to be used in the AMC Pacer, but development was pushed back.[19][20] American Motors designed the unique Pacer around the engine. By 1974, AMC had decided to purchase the General Motors Wankel instead of building an engine in-house.[21] Both General Motors and AMC confirmed the relationship would benefit in marketing the new engine, with AMC claiming that the General Motors' Wankel achieved good fuel economy.[22] However, General Motors' engines had not reached production when the Pacer was launched onto the market. The 1973 oil crisis played a part in frustrating the uptake of the Wankel engine. Rising fuel prices and talk about proposed US emission standards legislation also added to the concerns.

By 1974, General Motors R&D had not succeeded in producing a Wankel engine meeting both the emission requirements and good fuel economy, leading the company to consider cancelling the project. As General Motors managers were cancelling the Wankel project, the R&D team released only partly the results of their most recent research, which claimed to have solved the fuel economy problem, and building reliable engines with a lifespan above 530,000 miles. These findings were not taken into account when the cancellation order was issued. The cancellation of General Motors' Wankel project required AMC to reconfigure the Pacer to house its venerable AMC straight-6 engine driving the rear-wheels.[23]

In 1974, the Soviets created a special engine design bureau, which, in 1978, designed an engine designated as "VAZ-311". In 1980, the company commenced delivery of the VAZ-411 twin-rotor Wankel engine in VAZ-2106s and Lada cars, with about 200 manufactured. Most of the production went to the security services.[24][25] The next models were the VAZ-4132 and VAZ-415. Aviadvigatel, the Soviet aircraft engine design bureau, is known to have produced Wankel engines with electronic injection for aircraft and helicopters, though little specific information has surfaced.

The Wankel KKM motorcycle: The "A" marks one of the three apices of the rotor. The "B" marks the eccentric shaft and the white portion is the lobe of the eccentric shaft. The shaft turns 3 times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.

In the Wankel engine, the four strokes of a Otto cycle piston engine occur in the space between a three-sided symmetric rotor and the inside of a housing. In each rotor of the Wankel engine, the oval-like epitrochoid-shaped housing surrounds a rotor which is triangular with bow-shaped flanks (often confused with a Reuleaux triangle,[26] a three-pointed curve of constant width, but with the bulge in the middle of each side a bit more flattened). The theoretical shape of the rotor between the fixed corners is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively.[27] The symmetric curve connecting two arbitrary apexes of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).

The central drive shaft, called the "eccentric shaft" or "E-shaft", passes through the center of the rotor and is supported by fixed bearings.[28] The rotors ride on eccentrics (analogous to crankpins) integral to the eccentric shaft (analogous to a crankshaft). The rotors both rotate around the eccentrics and make orbital revolutions around the eccentric shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers.[27] The rotation of each rotor on its own axis is caused and controlled by a pair of synchronizing gears[28] A fixed gear mounted on one side of the rotor housing engages a ring gear attached to the rotor and ensures the rotor moves exactly 1/3 turn for each turn of the eccentric shaft. The power output of the engine is not transmitted through the synchronizing gears.[28] The force of gas pressure on the rotor (to a first approximation) goes directly to the center of the eccentric, part of the output shaft.

The easiest way to visualize the action of the engine in the animation at left is to look not at the rotor itself, but the cavity created between it and the housing. The Wankel engine is actually a variable-volume progressing-cavity system. Thus, there are three cavities per housing, all repeating the same cycle. Points A and B on the rotor and E-shaft turn at different speeds—point B circles three times as often as point A does, so that one full orbit of the rotor equates to three turns of the E-shaft.

As the rotor rotates orbitally revolving, each side of the rotor is brought closer to and then away from the wall of the housing, compressing and expanding the combustion chamber like the strokes of a piston in a reciprocating piston engine. The power vector of the combustion stage goes through the center of the offset lobe.

While a four-stroke piston engine completes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one-half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, the power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune; and higher than that of a four-stroke piston engine of similar physical dimensions and weight.

Wankel engines generally can sustain much higher engine revolutions than reciprocating engines of similar power output. This is due to the smoothness inherent in circular motion, and the absence of highly stressed parts such as crankshafts, camshafts or connecting rods. Eccentric shafts do not have the stress related contours of crankshafts. The maximum revolutions of a rotary engine is limited by tooth load on the synchronizing gears.[29] Hardened steel gears are used for extended operation above 7000 or 8000 rpm. Mazda Wankel engines in auto racing are operated above 10,000 rpm. In aircraft they are used conservatively, up to 6500 or 7500 rpm. However, as gas pressure participates in seal efficiency, racing a Wankel engine at high rpm under no load conditions can destroy the engine.

National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a four-stroke piston engine of 1.5 to 2 times the displacement. Some racing series have banned the Wankel altogether.[30]

Felix Wankel managed to overcome most of the problems that made previous rotary engines fail by developing a configuration with vane seals that had a tip radius equal to the amount of "oversize" of the rotor housing form, as compared to the theoretical epitrochoid, to minimize radial apex seal motion plus introducing a cylindrical gas-loaded apex pin which abutted all sealing elements to seal around the three planes at each rotor apex.[31]

Rotary engines have a thermodynamic problem not found in reciprocating four-stroke engines in that their "cylinder block" operates at steady state, with intake, compression, combustion, and exhaust occurring at fixed housing locations for all "cylinders". In contrast, reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts.

The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (maximum ~200 °C/400 °F) on a water-cooled Wankel engine. This gives a more constant surface temperature. The temperature around the spark plug is about the same as the temperature in the combustion chamber of a reciprocating engine. With circumferential or axial flow cooling, the temperature difference remains tolerable.[32][33][34][35]

During research in the 1950s and 1960s problems arose. For a while, engineers were faced with what they called "chattered marks" and "devil's scratch" in the inner epitrochoid surface. They discovered that the origin was in the apex seals reaching a resonating vibration, and solved the problem by reducing the thickness and weight of apex seals. Scratches disappeared after the introduction of more compatible materials for seals and housing coatings. Another early problem of the build-up of cracks in the stator surface was eliminated by installing the spark plugs in a separate metal insert in the housing instead of it being screwed directly into the block housing.[citation needed]. Toyota proved that substituting the leading site spark plug with a glow-plug improved low rpm, part load SFC by 7%, emissions and idle (SAE paper 790435). A later alternative solution to spark plug boss cooling was provided with a variable coolant velocity scheme for water-cooled rotaries which has had widespread use being patented by Curtiss-Wright,[36] with the last-listed for better air-cooled engine spark plug boss cooling. These approaches did not require a high conductivity copper insert but did not preclude its use. Ford tested an RCE with the plugs placed in the side plates, instead of in the housing working surface that was the usual way (Patent CA1036073, 1978).

Four-stroke reciprocating engines are less suitable for hydrogen. The hydrogen can misfire on hot parts like the exhaust valve and spark plugs. Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines. In a Wankel engine, this problem is circumvented by using a ceramic apex seal against a ceramic surface: there is no oil film to suffer hydrogenate attack. The piston shell must be lubricated and cooled with oil. This substantially increases the lubricating oil consumption in a four-stroke hydrogen engine.[37][38]

Increasing the displacement and power of a Wankel RCE by adding more rotors to a basic design is simple, but a limit may exist in the number of rotors, as power output is channeled through the last rotor shaft, with all the stresses of the whole engine present at this point. For engines with more than two rotors, the approach of coupling two bi-rotor sets by a serrate coupling between the two rotor sets has been tested successfully.

SPARCS in the UK found that idle stability and economy was obtained by supplying an ignitable mix to only one rotor in a multi rotor engine in a forced-air cooled rotor, similar to the later Norton designs.

Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion. While this places high demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials, such as exotic alloys and ceramics. With water cooling in a radial or axial flow direction, with the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable.

Among the alloys cited for Wankel RCE housing use are A-132, Inconel 625, and 356 treated to T6 hardness. Several materials have been used for plating the housing working surface, 'Nikasil' being one. Citroen, Mercedes-Benz, Ford, A P Grazen and others applied for patents in this field. For the apex seals, the choice of materials has evolved along with the experience gained, from carbon alloys, to steel, ferrotic, and others. The combination between housing plating and apex and side seals materials was determined experimentally, to obtain the best duration of both seals and housing cover. For the shaft, steel alloys with little deformation on load are preferred, the use of Maraging steel has been proposed for this.

Lead is a solid lubricant with leaded gasoline linked to a reduced wear of seals and housings. Leaded gasoline was the predominant type available in the first years of the Wankel engine's development. The first engines had the oil supply calculated with consideration of gasoline's lubricating qualities. Leaded gasoline was phased out, with Wankel engines needing an increased mix of oil in the gasoline to provide lubrication to critical engine parts. Experienced users advise, even in engines with electronic fuel injection, adding at least 1% of oil directly to gasoline as a safety measure in case the pump supplying oil to combustion chamber related parts fails or sucks in air. The SAE paper by D W Garside describes extensively Norton's choices of materials and cooling fins.

Several approaches involving solid lubricants were tested, and even the addition of MoS2, one cc per liter of fuel is advised (LiquiMoly). Many engineers agree that the addition of oil to gasoline as in old two-stroke engines is a safer approach for engine reliability than an oil pump injecting into the intake system or directly to the parts requiring lubrication. A combined oil-in-fuel plus oil metering pump is always possible.[39]

Early engine designs had a high incidence of sealing loss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines, carbon particles could become trapped between the seal and the casing, jamming the engine and requiring a partial rebuild. It was common for very early Mazda engines to require rebuilding after 50,000 miles (80,000 km). Further sealing problems arise from the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also causes uneven wear between the apex seal and the rotor housing, evident on higher mileage engines.[citation needed] The problem is exacerbated when the engine is stressed before reaching operating temperature. However, Mazda Wankel engines have solved these problems. Current engines have nearly 100 seal-related parts.[1]

The problem of clearance for hot rotor apexes passing between the axially closer side housings in the cooler intake lobe areas was dealt with by using an axial rotor pilot, radially inboard of the oils seals plus improved inertia oil cooling of the rotor interior ( C-W patents 3,261,542, C. Jones, 5/8/63, 3,176,915, M. Bentele, C.Jones. A.H. Raye. 7/2/62), and slightly "crowned" apex seals (different height in the center and in the extremes of seal).

Modern Wankel engines have fully sealed mainshaft cases. Many engines do not require oil changes as the oil is not contaminated by the combustion process.

The shape of the Wankel combustion chamber is resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine.[40] The combustion chamber shape also leads to relatively incomplete combustion of the air-fuel charge, with a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, as combustion temperatures are lower than in other engines, and also because of some inherent exhaust gas recirculation (EGR) in early engines. Sir Harry Ricardo showed in the 1920s that for every 1% increase in the proportion of exhaust gas in the admission mix, there is a 45 °F reduction in flame temperature. This allowed Mazda to meet the United States Clean Air Act of 1970 in 1973, with a simple and inexpensive 'thermal reactor' which is an enlarged chamber in the exhaust manifold. By decreasing the air-fuel ratio until unburned hydrocarbons (HC) in the exhaust would support combustion in the thermal reactor. Piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions. This inexpensive solution raised fuel consumption, which was already a weak point for the Wankel engine, at the same time that the oil crisis of 1973 raised the price of gasoline.

Mazda improved the fuel efficiency of the thermal reactor system by 40% by the time of introduction of the RX-7 in 1978. However, Mazda eventually shifted to the catalytic converter system.[28] According to the Curtiss-Wright research, the factor that controls the amount of unburned HC in the exhaust is the rotor surface temperature, with higher temperatures producing less HC.[41] Curtiss-Wright showed also that the rotor can be widened, keeping the rest of engine's architecture unchanged, thus reducing friction losses and increasing displacement and power output. The limiting factor for this widening being mechanical considerations, especially shaft deflection at high rotative speeds (SAE paper 710582). Quenching is the dominant source of HC at high speeds, and leakage at low speeds.[42]

Automobile Wankel rotary engines are capable of high speed operation. However, it was shown that an early opening of the intake port, longer intake ducts, and a greater rotor eccentricity can increase the amount of torque at low RPM. The shape and positioning of the rotor recess -combustion chamber- influences emissions and fuel economy, the MDR being chosen as a compromise, but which shape of the combustion recess gives better results in terms of fuel economy and exhaust emissions varies depending on the number and placement of spark plugs per chamber of the individual engine.[43]

In Mazda's RX-8 with the Renesis engine, fuel economy met California State requirements, including California's low emissions vehicle (LEV) standards. This was achieved by a number of innovations. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This solved the problem of the earlier ash buildup in the engine, and thermal distortion problems of side intake and exhaust ports. A scraper seal was added in the rotor sides, and by use of some ceramic-made parts in the engine. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing the exhaust port area. The side port trapped the unburned fuel in the chamber, decreased the oil consumption, and improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port Wankel engine are 35–50% less than those from the peripheral exhaust port Wankel engine, because of near zero intake and exhaust port opening overlap. Although peripheral ported RCEs have a better mean effective pressure, especially at high rpm and with a rectangular shaped intake port (SAE paper 288A).[44][45] However, the RX-8 was not improved to meet EuroV emission regulations and was discontinued in 2012.[46]

Mazda is still continuing development of the next generation of Wankel engines, the 16X. The company is researching engine laser ignition, eliminating spark plugs, and direct fuel injection to which the Wankel engine is suited. This leads towards a greater rotor eccentricity, equaling a longer stroke in a reciprocating engine, for better elasticity and low rpm torque. These innovations promise to improve fuel consumption and emissions.[47][48][49] To improve fuel efficiency further, Mazda is looking at using the Wankel as a range extender in series-hybrid cars and announced a prototype, the Mazda2 EV, for press evaluation in November 2013. This configuration improves fuel efficiency and emissions. As a further advantage, running a Wankel engine at a constant speed gives greater engine life. Keeping to a near constant, or narrow band, of revolutions eliminates, or vastly reduces, many of the disadvantages of the Wankel engine.[15]

It is approximately one third of the weight of a piston engine of equivalent power output

It is approximately one third of the size of a piston engine of equivalent power output

No reciprocating parts

Able to reach higher revolutions per minute than a piston engine

Operates with almost no vibration

Not prone to engine-knock

Cheaper to mass-produce as the engine contains fewer parts

Superior breathing, filling the combustion charge in 270 degrees of mainshaft rotation rather than 180 degrees in a piston engine

Supplies torques for about two thirds of the combustion cycle rather than one quarter for a piston engine

Wider speed range gives greater adaptability

It can use fuels of wider octane ratings

Does not suffer from "scale effect" to limit its size

On some Wankel engines the sump oil remains uncontaminated by the combustion process requiring no oil changes. The oil in the mainshaft is totally sealed from the combustion process. The oil for Apex seals and crankcase lubrication is separate. In piston engines the crankcase oil is contaminated by combustion blow-by through the piston rings.

Wankel engines are considerably lighter and simpler, containing far fewer moving parts than piston engines of equivalent power output. Valves or complex valve trains are eliminated by using simple ports cut into the walls of the rotor housing. Since the rotor rides directly on a large bearing on the output shaft, there are no connecting rods and no crankshaft. The elimination of reciprocating mass and the elimination of the most highly stressed and failure prone parts of piston engines gives the Wankel engine high reliability, a smoother flow of power, and a high power-to-weight ratio.

The surface-to-volume-ratio is so complex that a direct comparison cannot be made between a reciprocating piston engine and a Wankel engine. The flow velocity and the heat losses behave quite differently. Surface temperatures behave absolutely differently; the film of oil in the Wankel engine acts as insulation. Engines with a higher compression ratio have a worse surface-to-volume ratio. The surface-to-volume ratio of a diesel engine is much poorer than a gasoline engine, but diesel engines are well known for a higher efficiency factor. Thus, engines with equal power should be compared: a naturally aspirated 1.3-liter Wankel engine with a naturally aspirated 1.3-liter four-stroke reciprocating piston engine with equal power. But such a four-stroke engine is not possible and needs twice the displacement for the same power as a Wankel engine. When comparing the power-to-weight ratio or physical size to a similar output piston engine, the Wankel is superior.

The extra or "empty" stroke(s) should not be ignored, as a four-stroke cylinder produces a power stroke only every other rotation of the crankshaft. This doubles the real surface-to-volume ratio for the four-stroke reciprocating piston engine and the demand of displacement.[50][51] The Wankel, therefore, has higher volumetric efficiency and a lower pumping loss through the absence of choking valves.[52] Because of the quasi-overlap of the power strokes that cause the smoothness of the engine and the avoidance of the four-stroke cycle in a reciprocating engine, the Wankel engine is very quick to react to throttle changes and is able to quickly deliver a surge of power when the demand arises, especially at higher rpm. This difference is more pronounced when compared to four-cylinder reciprocating engines and less pronounced when compared to higher cylinder counts.

In addition to the removal of internal reciprocating stresses by virtue of the complete removal of reciprocating internal parts typically found in a piston engine, the Wankel engine is constructed with an iron rotor within a housing made of aluminium, which has a greater coefficient of thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as would be likely to occur in an overheated piston engine. This is a substantial safety benefit of use in aircraft. In addition, valves and valve trains that do not exist cannot burn out, jam, break, or malfunction in any way, again increasing safety.

A further advantage of the Wankel engine for use in aircraft is the fact that a Wankel engine generally has a smaller frontal area than a piston engine of equivalent power, allowing a more aerodynamic nose to be designed around it. The simplicity of design and smaller size of the Wankel engine also allows for savings in construction costs, compared to piston engines of comparable power output.

Wankel engines that operate within their original design parameters are almost immune to catastrophic failure. A Wankel engine that loses compression, cooling or oil pressure will lose a large amount of power and fail over a short period of time. It will, however, usually continue to produce some power during that time, allowing for a safer landing when used in aircraft. Piston engines under the same circumstances are prone to seizing or breaking parts that almost certainly results in catastrophic failure of the engine and instant full loss of power. For this reason, Wankel engines are very well suited to snowmobiles, which often take users into remote places where a failure could result in frostbite or death, and aircraft, where abrupt failure is likely to lead to a crash or forced landing in a remote place.

From the combustion chamber shape and features, the fuel ON requirements of Wankel RCEs are lower than in reciprocating ICEs, maximum road octane number requirements were 82 for a peripheral intake port RCE, and less than 70 for a side inlet port engine (SAE paper 720357), from the point of view of oil refiners this may be an industrial advantage in fuel production costs. ('Lubricant and Fuel Requirements and General Performance Data of Wankel Rotary Piston Engines', R D Behling and E Weise, BP, SAE paper 730048; 'A Refiner's Viewpoint on Motor Fuel Quality', W M Holaday and J Nappel, Socony-Vacuum Oil Co, SAE paper 430113).

Due to a 50% longer stroke duration than a reciprocating four-cycle engine, there is more time to complete the combustion. This leads to greater suitability for direct fuel injection and stratified charge operation. A Wankel rotary engine has stronger flows of air-fuel mixture and a longer operating cycle than a reciprocating engine, realizing concomitantly thorough mixing of hydrogen and air. The result is a homogeneous mixture, and no hot spots in the engine, which is crucial for hydrogen combustion.[53]

Many of the disadvantages are in ongoing research with some advances greatly reducing negative aspects of the engine. However, the current disadvantages of the Wankel engine in production are:[54]

Rotor sealing. This is still a problem as the engine housing has vastly different temperatures in each separate chamber section. The different expansion coefficients of the materials gives a far from perfect sealing. Additionally, both sides of the seals are being exposed to fuel, and the design does not allow for a dedicated lubrication system, as in two-stroke engines. In comparison, a piston engine has all functions of a cycle in the same chamber giving a more stable temperature for piston rings to act against; additionally, only one side of the piston in a (four-stroke) piston engine is being exposed to fuel, allowing for oil to lubricate the cylinders from the other side. To overcome the differences in temperatures between different regions of housing and side and intermediary plates, and the associated thermal dilatation inequities, the use of a heat pipe, transporting heat from the hot to the cold parts of engine, has been shown to reduce, in a small displacement, charge cooled rotor, air-cooled housing RCE, the maximal engine temperature from 231 °C to 129 °C, and the maximum difference from a hotter to a colder region of engine, from 159 °C to 18 °C. (SAE paper 2014-01-2160, by Wei Wu et al., University of Florida, USA).

Apex seal lifting. Centrifugal force pushes the apex seal onto the housing surface forming a firm seal. Gaps can develop between the apex seal and troichoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur. In low engine-rpm ranges, or under low-load conditions, gas pressure in the combustion chamber can cause the seal to lift off the surface, resulting in combustion gas leaking into the next chamber. Mazda has identified this problem and have developed a solution. By changing the shape of the troichoid housing, the seals remain flush to the housing. This points to using the engine at sustained higher revolutions eliminating apex seal lift off, in applications such as an electricity generator. In vehicles this leads to series-hybrid applications of the engine.[55]

Slow combustion. The combustion is slow as the combustion chamber is long, thin, and moving. The trailing side of the combustion chamber naturally produces a "squeeze stream" that prevents the flame from reaching the chamber trailing edge. Fuel injection in which fuel is injected towards the leading edge of the combustion chamber can minimize the amount of unburnt fuel in the exhaust.

Bad fuel economy. This is due to seals leakages, and the "difficult" shape of the combustion chamber, with poor combustion behavior and mean effective pressure at part load, low rpm. Meeting the emissions regulations requirements sometimes mandates a fuel-air ratio that is not the best for fuel economy. Acceleration and deceleration as in direct drive average driving conditions also affect fuel economy. Running the engine at a constant speed and load eliminates excess fuel consumption.[15]

Poor emissions. As unburnt fuel is in the exhaust stream, emissions requirements are difficult to meet. This problem may be overcome by implementing direct fuel injection into the combustion chamber. The Freedom Motors Rotapower Wankel engine, which is not yet in production, met the ultra low California emissions standards.[citation needed] The Mazda Renesis engine, with both intake and exhaust side ports, suppressed the loss of unburned mix to exhaust formerly induced by port overlap.[56]

Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the opposite is true. As well as the rotor apex seals evident in the conceptual diagram, the rotor must also seal against the chamber ends.

Piston rings are not perfect seals: each has a gap to allow for expansion. The sealing at the Wankel apexes is less critical, as leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the crankcase. Although sealing has improved over the years, the less than effective sealing of the Wankel, which is mostly due to lack of lubrication, is still a factor reducing its efficiency.[57] Comparison tests have shown that the Mazda rotary powered RX-8 sports car may use more fuel than a heavier vehicle powered by larger displacement V-8 engines for similar performance results.[58]

The fuel-air mixture cannot be pre-stored as there are consecutive intake cycles. The Wankel engine has a 50% longer stroke duration than a piston engine. The four Otto cycles last 1080° for a Wankel engine (three revolutions of the output shaft) versus 720° for a four-stroke reciprocating piston engine, but the four strokes are still the same proportion of the total.

There are various methods of calculating the engine displacement of a Wankel. The Japanese regulations for calculating displacements for engine ratings use the volume displacement of one rotor face only, and the auto industry commonly accepts this method as the standard for calculating the displacement of a rotary. When compared by specific output, however, the convention results in large imbalances in favor of the Wankel motor, an early approach was rating displacement of each rotor as two times the chamber.

Wankel rotary engine and piston engine displacement and corresponding power output can more accurately be compared by displacement per revolution of the eccentric shaft. A calculation of this form dictates that a two-rotor Wankel displacing 654 cc per face will have a displacement of 1.3 liters per every rotation of the eccentric shaft (only two total faces, one face per rotor going through a full power stroke) and 2.6 liters after two revolutions (four total faces, two faces per rotor going through a full power stroke). The results are directly comparable to a 2.6-liter piston engine with an even number of cylinders in a conventional firing order, which will likewise displace 1.3 liters through its power stroke after one revolution of the crankshaft, and 2.6 liters through its power strokes after two revolutions of the crankshaft. A Wankel rotary engine is still a four-stroke engine and pumping losses from non-power strokes still apply, but the absence of throttling valves and a 50% longer stroke duration result in a significantly lower pumping loss compared to a four-stroke reciprocating piston engine. Measuring a Wankel rotary engine in this way more accurately explains its specific output, as the volume of its air fuel mixture put through a complete power stroke per revolution is directly responsible for torque and thus power produced.

The trailing side of the rotary engine's combustion chamber develops a squeeze stream which pushes back the flamefront. With the conventional one or two-spark-plug system and homogenous mixture, this squeeze stream prevents the flame from propagating to the combustion chamber's trailing side in the mid and high engine speed ranges, Mazda engineers described the full process in 'Combustion characteristics of Rotary Engines', K Yamamoto et al., SAE paper 720357. Kawasaki addressed this problem in their US patent nº 3848574, and Toyota (SAE paper 790435) obtained a 7% economy improvement by placing a glow-plug in the leading site and using Reed-valves in intake ducts. This poor combustion in the trailing side of chamber is one of the reasons why there is more carbon monoxide and unburnt hydrocarbons in a Wankel's exhaust stream. A side-port exhaust, as is used in the Mazda Renesis, avoids one of the causes of this because the unburned mixture cannot escape. The Mazda 26B avoided this issue through a three spark-plug ignition system. (At the 24 Hours of Le Mans endurance race in 1991 the 26B had significantly lower fuel consumption than the competing reciprocating piston engines. All competitors had the same amount of fuel available due to the Le Mans limited fuel quantity rule.)[59] An inventor proposed that a narrow linear opening for spark plugs in the housing, instead of the round hole, would improve volumetric efficiency, fuel economy and emissions; he claims to have proved this by recording a measured reduction in the exhaust gas temperature. As in early reciprocating engines, a lower effective compression ratio is linked to higher exhaust gas temperatures. (YouTube: 'Rotary Engine Breakthrough')

A peripheral intake port gives the highest mean effective pressure, however, side intake porting produces a more steady idle,[60] as it helps to prevent blow-back of burned gases into the intake ducts which cause "misfirings": alternating cycles where the mixture ignites and fails to ignite; peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was linked also to worse idle stability and part-load performance. Early work from Toyota (SAE paper 790435) led to the addition of a fresh air supply to the exhaust port and proved also that a Reed-valve in the intake port or ducts (SAE paper 720466, Ford 1979 patent CA1045553) improved the low rpm and partial load performance of Wankel RCEs, by preventing blow-back of exhaust gas into the intake port and ducts, and reducing the misfiring-inducing high EGR, at the cost of a small loss of power at top rpm; this is according to David W. Garside, the developer of the Norton rotary engine, who proposed that an earlier opening of the intake port before top dead center (TDC) and longer intake ducts improved low rpm torque and elasticity of RCEs, also described in K Yamamoto's books. Elasticity is also improved with a greater rotor eccentricity, analogous to a longer stroke in a reciprocating engine. Wankel engines operate better with a low pressure exhaust system, higher exhaust backpressure reducing mean effective pressure, more severely in peripheral intake port engines. The Mazda RX-8 Renesis engine improved performance by doubling the exhaust port area respect to earlier designs, and there is specific work about the effect of intake and exhaust piping configuration on RCEs performance (SAE papers by Ming-June Hsieh et al.).

All Mazda-made Wankel rotaries, including the Renesis found in the RX-8, burn a small quantity of oil by design, metered into the combustion chamber to preserve the apex seals. Owners must periodically add small amounts of oil, thereby increasing running costs. Some sources (rotaryeng.net) claim that better results come with the use of an oil-in-fuel mixture rather than an oil metering pump. Liquid cooled engines require a mineral multigrade oil for cold starts, and RCEs need a warm-up time before full load operation as reciprocating engines do. All engines exhibit oil loss, however the rotary engine is engineered with a sealed motor, unlike a piston engine that has a film of oil that splashes on the walls of the cylinder to lubricate them, hence an oil "control" ring. No-oil-loss engines have been developed eliminating much of the oil lubrication problems.[61]

As the rotor's apex seals pass over the spark plug hole, compressed charge can be lost from the charge chamber to the exhaust chamber, entailing fuel in the exhaust, reducing efficiency, and giving high emissions. This may be overcome by using laser ignition, eliminating traditional spark plugs, which may give a narrow slit in the motor housing the rotor apex seals can fully cover with no loss of compression from one chamber to another. The laser plug can fire its spark through the narrow slit. T Kohno et al. from Toyota -SAE paper 790435- proved that installing a glow-plug in the leading site improved in 7% part load and low rpm fuel economy. Direct fuel injection of which the Wankel engine is suited, combined with laser ignition in single or multiple laser plugs, will enhance the motor even further reducing the disadvantages.[47][48][49]

In the racing world, Mazda has had substantial success with two-rotor, three-rotor, and four-rotor cars. Private racers have also had considerable success with stock and modified Mazda Wankel-engine cars.[62]

The Sigma MC74 powered by a Mazda 12A engine was the first engine and only team from outside Western Europe or the United States to finish the entire 24 hours of the 24 Hours of Le Mansrace, in 1974. Mazda is the only team from outside Western Europe or the United States to have won Le Mans outright and the only non-piston engine ever to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (2,622 cc or 160 cu in—actual displacement, rated by FIA formula at 4,708 cc or 287 cu in).

Formula Mazda Racing features open-wheel race cars with Mazda Wankel engines, adaptable to both oval tracks and road courses, on several levels of competition. Since 1991, the professionally organized Star Mazda Series has been the most popular format for sponsors, spectators, and upward bound drivers. The engines are all built by one engine builder, certified to produce the prescribed power, and sealed to discourage tampering. They are in a relatively mild state of racing tune, so that they are extremely reliable and can go years between motor rebuilds.[63]

The Malibu Grand Prix chain, similar in concept to commercial recreational kart racing tracks, operates several venues in the United States where a customer can purchase several laps around a track in a vehicle very similar to open wheel racing vehicles, but powered by a small Curtiss-Wright rotary engine.

In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft may be used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the Mazda's production three-rotor 20B-REW engine, as well as many low volume production race engines. The C-111-2 4 Rotor Mercedes-Benz eccentric shaft for the KE Serie 70, Typ DB M950 KE409 is made in one piece. Mercedes-Benz used split bearings.

The small size and attractive power to weight ratio of the Wankel engine attracted motorcycle manufacturers. The first Wankel-engined motorcycle was the 1960 'IFA/MZ KKM 175W' built by German motorcycle manufacturer MZ licenced from NSU.[64]

In 1972, Yamaha introduced the RZ201 at the Tokyo Motor Show, a prototype with a Wankel engine, weighing 220 kg and producing 60 hp from a two-rotor 660 cc engine (US patent N3964448). Kawasaki presented also in 1972 its two-rotor Kawasaki X99 RCE prototype (US patent N 3848574, and also 3991722), both Yamaha and Kawasaki claimed having solved all problems previously found in Wankel RCEs, fuel economy, exhaust emissions, and engine duration, but none entered the stage of production.

From 1974 to 1977, Hercules produced a limited number of motorcycles powered by Wankel engines, whose production was discontinued because of a failure to attain the necessary number of motorcycles sold by month to reach profitability by 27 units.[65]

The Suzuki RE5 single-rotor motorcycle was produced from 1975 to 1976. It proved to be a complex design, with liquid cooling and oil cooling, and multiple lubrication and carburetor systems, which caused problems that were solved after the first series units. It worked well and was smooth, but being rather heavy and having a modest 62 bhp power output, it did not sell well.[66]

Dutch motorcycle importer and manufacturer Van Veen produced small quantities of their dual-rotor Wankel-engined OCR-1000 between 1978 and 1980, using surplus Comotor engines.

In the early 1980s, David Garside's BSA twin-rotor engine reached production at Norton as the air-cooled twin-rotor Norton Classic. The Classic was followed by the liquid-cooled Norton Commander, with the aim of making the engine less sensitive to the user's habits in warm-up, and the Interpol2, a police version. (These machines used motor tooling and blank apex seals).[67] The Norton engine later formed the basis for the British MidWest AE series aero-engines, then the Diamond, and subsequently the Austro engine. Norton used a Wankel engine in several road models including the Norton F1, F1 Sports, RC588, RCW588, and NRS588. Norton has proposed a new 588 cc twin-rotor model called the "NRV588" and a 700 cc version called the "NRV700".[68] A former mechanic at Norton, Brian Crighton, started developing his own rotary engined motorcycles line named "Roton", whose products won several local Australian races.

Despite success in racing, no motorcycles powered by Wankel engines have been produced for sale to the general public for road use since 1992.

In principle, a Wankel engine should be ideal for light aircraft, as it is light, compact, almost vibrationless and has a high power-to-weight ratio. Further aviation benefits of a Wankel engine include:

Rotors cannot seize, since rotor casings expand more than rotors;

A Wankel engine is less prone to the serious condition known as "engine-knock", which can destroy the plane's engines in mid-flight.

A Wankel is not susceptible to "shock-cooling" during descent;

A Wankel does not require an enriched mixture for cooling at high power;

Having no reciprocating parts, there is less vulnerability to damage when the engines revolves higher than the designed maximum running operation. The limit to the revolutions is the strength of the main bearings.

Unlike the case with some cars and motorcycles, a Wankel aero-engine will be sufficiently warm before full power is asked of it because of the time taken for pre-flight checks. A Wankel aero-engine spends most of its operational time at high power outputs, with little idling. This makes ideal the use of peripheral ports. An advantage is that modular engines with more than two rotors are feasible. If icing of any intake tracts is an issue, there is plenty of waste engine heat available to prevent icing.

The first Wankel rotary-engine aircraft was the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, basically a powered Schweizersailplane, in the late 1960s. The plane was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine; the same engine model was also flown in a Cessna Cardinal and other airplanes and a helicopter.[11][69][70] In Germany in the mid-1970s, a pusher ducted fan airplane powered by a modified NSU multi-rotor Wankel engine was developed in both civilian and military versions, Fanliner and Fantrainer.

In roughly the same timeframe as the first experiments with full-scale aircraft powered with Wankel engines, model aircraft-sized versions were pioneered by a combine of the well-known Japanese O.S. Engines firm and the then-extant German Graupner aeromodeling products firm, under license from NSU/Auto-Union. By 1968 the first prototype air-cooled, single-rotor glow plug-ignition, methanol fueled 4.9 cm3 displacement OS/Graupner model Wankel engine was running, and was produced in at least two differing versions from 1970 to the present day, solely by the O.S. firm since Graupner's demise in 2012.[71]

Aircraft Wankels have been taken up with the advantages over other engines being exploited. Wankels are increasingly being found in roles where the compact size, high power-to-weight ratio and quiet operation is important, notably in drones and unmanned aerial vehicles. Many companies and hobbyists adapt Mazda rotary engines (taken from automobiles) to aircraft use; others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for this purpose.[72][73] One such use are the "Rotapower" engines in the Moller Skycar M400. Another example of purpose built aircraft rotaries are Austro Engine's 55 hp (40.4 kW) AE50R (certified) and 75 hp (55 kW) AE75R (under development) both appr. 2 hp/kg.[74]

Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft, such as the ARV Super2 which can be re-engined with the British MidWest AE series aero-engine. Most are Mazda 12A and 13B automobile engines, converted to aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to 300 horsepower (220 kW) at a fraction of the cost of traditional engines. These conversions first took place in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these were a failure due to design or manufacturing flaws.[citation needed]

Peter Garrison, contributing editor for Flying magazine, has said that "In my opinion, however, the most promising engine for aviation use is the Mazda rotary."[75] Mazdas have indeed worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is producing FAA-certified alternatives to the standard reciprocating engines that power most small general aviation aircraft. Mistral Engines, based in Switzerland, developed purpose-built rotaries for factory and retrofit installations on certified production aircraft. The G-190 and G-230-TS rotary engines were already flying in the experimental market, and Mistral Engines hoped for FAA and JAA certification by 2011. As of June 2010, G-300 rotary engine development ceased, with the company citing a need for cash flow to complete development.[76]

Mistral claims to have overcome the challenges of fuel consumption inherent in the rotary, at least to the extent that the engines are demonstrating specific fuel consumption within a few points of reciprocating engines of similar displacement. While fuel burn is still marginally higher than traditional engines, it is outweighed by other beneficial factors.[77][78]

At the price of increased complication for a high pressure diesel type injection system, fuel consumption in the same range as small pre-chamber automotive and industrial diesels has been demonstrated with Curtiss-Wright's stratified charge multi-fuel engines, while preserving the aforementioned Wankel rotary advantages[79] Unlike a piston and overhead valve engine, there are no valves which can float at higher rpm causing loss of performance. The Wankel by design has fewer moving part,and no cylinder head, making it a more effective design with higher rpm potential.[80]

Since Wankel engines operate at a relatively high rotational speed with relatively low torque, propeller aircraft must use a propeller speed reduction unit (PSRU) to maintain propellers within the designed speed range. Experimental aircraft with Wankel engines use PSRUs: for instance, the MidWest twin-rotor engine has a 2.95:1 reduction gearbox. The rotational shaft speed of a Wankel engine is high compared to reciprocating piston designs. Only the eccentric shaft spins fast, while the rotors turn at exactly one-third of the shaft speed. If the shaft is spinning at 7,500 rpm, the rotors are turning at a much slower 2,500 rpm.

Pratt & Whitney Rocketdyne have been commissioned by DARPA to develop a diesel Wankel engine for use in a prototype VTOLflying car called the "Transformer".[81] The engine, based on an earlier UAV diesel Wankel concept called "Endurocore",[82] plans to utilize Wankel rotors of varying sizes on a shared eccentric shaft to increase efficiency.[83] The engine is claimed to be a 'full-compression, full-expansion, diesel-cycle engine'. An October 28, 2010 patent from Pratt & Whitney Rocketdyne, describes a Wankel engine superficially similar to Rolls-Royce's earlier prototype that required an external air compressor to achieve high enough compression for diesel-cycle combustion.[84][85] The design differs from Rolls-Royce's diesel Wankel mainly by proposing an injector both in the exhaust passage between the combustor rotor and expansion rotor stages, and an injector in the expansion rotor's expansion chamber, for 'afterburning'.

In 2013 e-Go aeroplanes, based in Cambridge, United Kingdom, announced that their new single-seater canard aircraft, the winner of a design competition to meet the new UK single-seat deregulated category, will be powered by a Wankel engine from Rotron Power Ltd, a specialist manufacturer of advanced rotary engines for unmanned aeronautical vehicle (UAV) applications. The aircraft is expected to deliver 100 kts cruise speed from a 30 hp Wankel engine, with a fuel economy of 75mpg using standard MOGAS.

The DA36 E-Star, an aircraft designed by Siemens, Diamond Aircraft and EADS, employs a series hybrid powertrain with the propeller being turned by a Siemens 70 kW (94 hp) electric motor. The aim is to reduce fuel consumption and emissions by up to 25 percent. An onboard 40 hp (30 kW) Austro Engines Wankel rotary engine and generator provides the electricity. A propeller speed reduction unit is eliminated. The electric motor uses electricity stored in batteries, with the engines off, to take off and climb reducing sound emissions. The series-hybrid powertrain using the Wankel engine reduces the weight of the plane by 100 kilograms from its predecessor. The DA36 E-Star first flew in June 2013, making this the first ever flight of a series-hybrid powertrain. Diamond Aircraft state that the technology using Wankel engines is scalable to a 100-seat aircraft.[87][88]

Structure of a series-hybrid vehicle. The grey square represents a differential gear. An alternative arrangement (not shown) is to have electric motors at two or four wheels.

Mazda2 EV prototype

Due to the compact size and the high power to weight ratio of a Wankel engine, a number have been proposed for electric vehicles as range extenders with a number of concept cars displaying a series hybrid powertrain arrangement. A Wankel engine used only as a generator has packaging and weight distribution advantages, maximizing interior passenger and luggage space when used in a vehicle. The engine/generator may be at one end of the vehicle with the electric driving motors at the other connected only by thin light cables. In 2010 Audi revealed a prototype series-hybrid electric car, the A1 e-tron, that incorporated a small 250 cc Wankel engine running at 5,000 rpm recharging the car's batteries as needed, and providing electricity directly to the electric driving motor.[89][90] In 2010 FEV Inc revealed that in their prototype electric version of the Fiat 500 a Wankel engine would also be used as a range extender.[91]Valmet Automotive of Finland in 2013 revealed a prototype incorporating a Wankel powered series-hybrid powertrain car named the EVA, utilizing an engine manufactured by the German company Wankel SuperTec.[92]

Mazda of Japan ceased production of Wankel engines in their model range in 2012, leaving the motor industry world-wide with no production cars using the engine. However Mazda announced they are to use an updated Wankel engine, the RX-16, with laser ignition and direct fuel injection being researched in a series-hybrid arrangement.[93] Mr Takashi Yamanouchi, the global CEO of Mazda stated, "The rotary engine has very good dynamic performance, but it's not so good on economy when you accelerate and decelerate. However, with a range extender you can use a rotary engine at a constant 2,000rpm, at its most efficient. It's compact, too."[15] No Wankel engine in this arrangement, as yet has made it into production vehicles or planes. However, in November 2013 Mazda announced a series-hybrid prototype car to the motoring press, the Mazda2 EV using a Wankel engine as a range extender. The engine is a tiny, almost inaudible, single-rotor 330cc unit generating 30 bhp at 4,500rpm maintaining a continuous electric output of 20 kW. The engine is located under the rear luggage floor.[94][95][96]

Small Wankel engines are being found increasingly in other applications, such as go-karts,[97][98]personal water craft and auxiliary power units for aircraft.[99][100] Kawasaki patented also a mixture cooled RCE engine (US patent 3991722). Yanmar Diesel and Dolmar-Sachs had a rotary engine chain saw (SAE paper 760642) and outboard boat engines, and the French Outils Wolf, a Wankel RCE powered lawnmower (Rotondor), with the rotor in a horizontal position and no seals in the down side, for production costs savings. The Graupner/O.S. 49-PI is a 1.27 hp (947 W) 5 cc Wankel engine for model airplane use which has been in production essentially unchanged since 1970; even with a large muffler, the entire package weighs only 380 grams (13.4 ounces).[101][102]

The simplicity of the Wankel makes it well-suited for mini, micro, and micro-mini engine designs. The Microelectromechanical systems (MEMS) Rotary Engine Lab at the University of California, Berkeley, has previously undertaken research towards the development of Wankel engines of down to 1 mm in diameter with displacements less than 0.1 cc. Materials include silicon and motive power includes compressed air. The goal of such research was to eventually develop an internal combustion engine with the ability to deliver 100 milliwatts of electrical power; with the engine itself serving as the rotor of the generator, with magnets built into the engine rotor itself.[103][104] Development of the miniature Wankel engine stopped at UC Berkeley at the end of the DARPA contract. Miniature Wankel engines struggled to maintain compression due to sealing problems, similar to problems observed in the large scale versions. In addition, miniature engines suffer from an adverse surface to volume ratio causing excess heat losses; the relatively large surface area of the combustion chamber walls transfers away what little heat is generated in the small combustion volume resulting in quenching and low efficiency.

Ingersoll-Rand built the largest ever Wankel engine which was available between 1975 and 1985 producing 1,100 hp (820 kW) with two rotors. A one rotor version was available producing 550 hp (410 kW). The displacement per rotor was 41 liters with each rotor approximately one meter in diameter. The engine was derived from a previous, unsuccessful Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines: the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, and thereby limiting the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm and the use of natural gas as fuel; this was particularly well chosen, since one of the major uses of the engine was to drive compressors on natural gas pipelines.[105]

Yanmar Diesel of Japan produced some small, charge-cooled rotor rotary engines for uses such as chainsaws and outboard engines,[106] some of their contributions are the LDR (rotor recess in the leading edge of combustion chamber) engines having better exhaust emissions profiles, and that reed-valve controlled intake ports improved part-load and low rpm performance.[107]

In 1971 and 1972, Arctic Cat produced snowmobiles powered by 303 cc Wankel rotary engines manufactured by Sachs in Germany.

In the early 1970s Johnson and other brands sold Snowmobiles powered by 35 or 45 HP Wankel engines designed and built by OMC.

In addition for use as an internal combustion engine, the basic Wankel design has also been used for gas compressors, and superchargers for internal combustion engines, but in these cases, although the design still offers advantages in reliability, the basic advantages of the Wankel in size and weight over the four-stroke internal combustion engine are irrelevant. In a design using a Wankel supercharger on a Wankel engine, the supercharger is twice the size of the engine.

The Wankel design is used in the seat belt pre-tensioner system[108] of some Mercedes-Benz[109] and Volkswagen[110] cars. When the decelerationsensors sense a potential crash, small explosive cartridges are triggered electrically and the resulting pressurized gas feeds into tiny Wankel engines which rotate to take up the slack in the seat belt systems, anchoring the driver and passengers firmly in the seat before a collision.[111]

P V Lamarque, "The Design of Cooling Fins for Motor-Cycle Engines", The Institution of Automobile Engineers Magazine, London, March 1943 issue, and also in "The Institution of Automobile Engineers Proceedings", XXXVII, Session 1942–1943, pp. 99–134 and 309–312.